1. Field of the Invention
The present invention relates to a wire saw for cutting wafers from a workpiece, in particular a wire saw for producing semiconductor wafers. The wire saw comprises a cutting head in which at least two wire-guiding rollers are rotatably mounted in each case between a moveable bearing and a fixed bearing, with adjacent wire-guiding rollers, together with a wire wound around them, forming a wire web. The wire web is used with a sawing suspension and functions as a cutting tool. A sawing suspension may be dispensed with if a sawing wire having bonded cutting particles is used. The invention also relates to a repair station and to a maintenance and testing station for a wire-guiding-roller unit and to a method of exchanging the wire-guiding-roller unit.
2. The Prior Art
A wire saw for cutting wafers and having a cutting head in which the wire-guiding rollers are mounted between a moveable bearing and a fixed bearing in this cutting head, is described, in EP-733 429 A1. This wire saw can be used to cut a multiplicity of semiconductor wafers of a specific thickness from a piece of crystal during a cutting operation. The thickness of the semiconductor wafers is determined by the distance between the windings of the sawing wire in the wire web. This distance, in turn, is essentially predetermined by grooves which are located in the coating of the wire-guiding rollers and in which the wire runs. The wire saw also has a heating/cooling system for controlling the temperature of the wire-guiding rollers and of the moveable bearing and fixed bearing adjacent thereto. In this way, undesired sawing-wire displacement as a result of heat expansion of machine parts can be minimized.
Occasionally, it is necessary to change the coating of the wire-guiding roller or to change the wire-guiding roller itself. For example, change may be needed when the cutting results deteriorate as a consequence of the coating being worn. Change may also be necessary if it is intended that the thickness of wafers which are to be cut subsequently should be modified to a new value. Deterioration of the cutting result may also indicate damaged bearings.
If the coating or the bearings are changed, it is necessary for the wire-guiding roller or the bearings to be removed from the cutting head. Up until now, this has been very expensive and has resulted in the wire saw being out of service and at a standstill for long periods of time. Once a new wire-guiding roller or new or repaired bearings have been introduced, the wire saw then still requires additional time. During this certain period of time, the wire saw is test run and is brought to the operating temperature before stable operation is possible. After this, a test workpiece then usually has to be sawed before the actual production of the wafers can proceed.